Puree watermelon in a food processor or blender. Add to a pitcher with lime juice and agave nectar or superfine sugar, stirring to blend; cover and refrigerate until very cold. (Can be done up to 8 hours ahead.)

Serve in glasses filled with ice, topped with a splash of seltzer and a lime wedge.

A Part of Hearst Digital Media
Woman's Day participates in various affiliate marketing programs, which means we may get paid commissions on editorially chosen products purchased through our links to retailer sites.